Every off-line SPS includes two sections: a conversion circuit and a supporting section. The conversion circuit includes all components that are necessary to perform the conversion, as if these components were ideal. The conversion circuit may be therefore just slightly simpler than one used in an actual SPS that operates without any load but is capable of delivering a full output power for a brief period of time. The conversion circuit comprises input capacitor, power switch, power transformer, output rectifier, etc. Furthermore, the circuit comprises components that ensure a safe and proper operation of the SPS. Those components include fuse, inrush current limiter, transient protector, power switch driver, primary current sensor, feedback circuit incorporating frequency compensation and line isolation, etc.
The supporting section exists only due to imperfections of the components used in the conversion circuit, in particular power devices. Basically, the supporting section is used to clean up the mess caused by the switching operation of the SPS delivering the full output power. The supporting section includes input and output filters, heat sinks, snubbers, shields, magnetic ducts, ferrite beads, etc.
Obviously, a common goal is to minimize the supporting section. One tendency is to use better components. For example, power semiconductors that switch faster and conduct better result in a higher efficiency of the SPS, reduced drive currents and smaller heat sinks. They also allow a higher switching frequency that results in smaller inductive and capacitive components. However, those better semiconductors may not be acceptable in a price sensitive environment.
Another tendency is to incorporate the parasitic elements into the conversion circuit rather than merely fight the consequences of their existence. Sometimes only a partial integration is possible. For example, on-resistance of the power switch can be used for the current sensing. However, the on-resistance does still contribute to the power dissipation of the switch. In other cases, a complete integration of the parasitic elements is possible. For example, some resonant type SPSs use the leakage inductance of the power transformer as the basic component of the resonant network.
Today's low power SPSs are dominated by forward and flyback SPSs. The input voltage is applied across the primary winding of the power transformer when one or two switches are closed. In one switch SPSs the energy stored in the leakage inductance of the power transformer is transferred to a passive snubber network, when the switch opens. A largest possible portion of the leakage energy is dissipated as heat to minimize another portion that is inevitably converted into EMI/RFI.
Forward SPSs are also often used in a medium power range. One switch forward SPS may comprise a reset winding. Voltage ratings of the switch are twice the input voltage. These ratings are cut in half in two switch forward SPS. Respective rectifiers have equal voltage ratings and peak current capabilities. When the switch or switches turn off, the energy stored in the transformer is returned to the input source. However, the input current responds in the worst possible manner as it changes its polarity at the peak. The input capacitor is then recharged by the current spike. This results in a voltage ripple that appears across the input of the SPS. Very often the current spike causes high frequency oscillations that are very difficult to filter. The circuit complexity and cost are degraded.
Generally, resonant type SPSs are inherently inferior to square wave type SPSs. In the resonant type SPSs a sinusoidal voltage arid/or current is developed. That, however, is associated with a considerable interval, usually near zero crossing of the respective sinusoidal signal, while a minimal or no energy transfer takes place. Therefore, the resonant SPSs require power semiconductors having higher current and/or voltage ratings. An output capacitor carries a high ripple current. By contrast, in square wave type SPSs a maximum energy transfer is accomplished instantaneously.
The resonant type SPSs have other inherent flaws. The resonant or switching frequency is determined by additional LC components, or an additional capacitor and the leakage inductance of the power transformer. These components are fixed and so is the turn-on or turn-off time. Pulse frequency modulation (PFM) rather than pulse width modulation (PWM) or other preferred switching method is often used to accomplish the regulation. A minimum load may be necessary to avoid large variations of the switching frequency. Moreover, the maximum switching frequency can be quite high if it is determined by the leakage inductance. The resonant type SPSs are preferred in special applications, such a high voltage conversion. General purpose SPS are costly and constitute a small fraction of all SPSs manufactured today.
SPSs have a tremendous drawback comparing with linear power supplies. Any SPS is basically a high power oscillator that is rich with switching harmonics. Moreover, if the AC line is used as the input source, numerous line frequency harmonics are created. The SPSs are therefore excellent sources of both conducted and radiated EMI. The power is not transferred to the output during the time allowed for charging or resetting the transformer. Therefore, the output ripple voltage delivered to the load is significantly larger.
The leakage inductance of the power transformer is often the key parasitic element and the largest single factor in degrading the performance of an off-line SPS. An ideal transformer has no leakage inductance because magnetic fields generated by the primary winding are entirely contained by the core and coupled completely to the secondary winding. The leakage inductance represents magnetic fields that do not couple with both the primary and secondary windings. The leakage inductance can be also expressed as a sum of the primary leakage inductance and the secondary leakage inductance multiplied by the square of the turns ratio.
The inductive leakage energy is transferred to a snubber capacitor. A diode is employed for charging the capacitor to a peak voltage. However, parasitic capacitances are charged to that voltage as well. As the diode ceases the conduction, the respective resonant circuit continues to oscillate at a very high frequency. Obviously, the oscillations appear directly at the input. They are also applied to the collector or drain of the power switch, usually its case, and further a heat sink. Moreover, the oscillations are also transferred to all other windings and are thus applied to all components connected thereto. The number and size of filters, snubbers, shields and beads determine merely a distribution of the leakage energy that is wasted in form of heat and EMI/RFI.
The value of the leakage inductance is determined primarily by physical dimensions. For example, safety regulations of various countries require specified amounts of spacing and insulation between the windings. Turns ratio of a typical off-line transformer makes it difficult for the secondary winding to uniformly cover the primary winding. Furthermore, economics may be an overriding consideration since some core shapes are easier and less expansive to wind than others. Transformer design becomes a tradeoff between these and other factors that always result in a significant amount of the leakage inductance.